Displaced Iraqis from the Yazidi community cross the Syrian-Iraqi border in northern Iraq. Photograph: Ahmad al-Rubaye/AFP/Getty

A team of US marines and special forces landed on Mount Sinjar in Iraq on Wednesday to assess options for a potential rescue of Yazidi civilians threatened by Islamic extremists and worn down by lack of food.

The personnel flew in on V-22 Osprey tilt-rotor aircraft that can land vertically, joining a small number of American special forces who, the Guardian has been told, had been on the mountain for some days. That team had been assessing the military and humanitarian situation and guiding US air strikes against Islamic State (Isis) fighters encircling the mountain.

A handful of British SAS soldiers were also in the area to “gather intelligence”, a British official said. The developments were the first confirmation that international forces were on Mount Sinjar.

Fleeing Yazidis have reported seeing small teams of American soldiers high on the northern flank. “We weren’t allowed to go near them,” said a man from Sinjar who was airlifted from the former base. “They were being guarded by the Kurds.”

The team that landed on Mount Sinjar on Wednesday conducted a reconnaissance mission before returning to the Kurdish regional capital, Irbil, officials said. “Today a team of fewer than 20 US personnel conducted an assessment of the situation on Mount Sinjar. All personnel have returned safely to Irbil by military air,” a US defence official said, on condition of anonymity.

The US later announced that a drone strike destroyed armed truck operated by Islamic militants in the area, the latest of several strikes conducted against Isis positions in recent days.

In Washington, the Obama administration was forced to defend itself against accusations of mission creep after admitting that it was considering a direct role for US forces in creating a “humanitarian corridor” for an evacuation of the Yazidis.

“[The president] is open to recommendations in which the United States is helping to facilitate the removal of these people from the mountain,” said the National Security Council spokesman, Ben Rhodes.

“If there are additional things we can do as part of an effort to move people off the mountain he will certainly review those options,” he added.

Pressed on whether this would contradict Obama’s promise to avoid a combat role for US ground troops if this evacuation mission came under attack, Rhodes added: “There are dangers involved in any military operation … but [the president] is confident that we can have a limited military objective.”

The US on Tuesday announced the deployment of an additional 129 forces to Iraq to assess the crisis and provide a report to Obama. That team arrived in Irbil on Wednesday, a day of sharply increased western commitment to the Kurds to help them fight off the extremist Isis movement.

The US, UK and Kurds have been conducting air drops of food and water for the besieged civilians, mostly ethnic Kurds of the Yazidi faith, who fled to the mountain 12 days ago to escape Isis militants who see them as non-believers. The UK is also contributing Chinook helicopters to help airlift some of the trapped population, which includes large numbers of women and children.

One of the issues the US marines will have to decide is whether the escape route should be by land or air.

Aid officials in the region said that any operation to open a land route would probably require significantly greater number of soldiers, American and peshmerga than are currently available in the region, as they would have to fight their way across Isis controlled territory to reach Mount Sinjar. That would represent a political problem for Washington. US officials have stressed that the American troops in Iraq will not be involved in combat.

An airlift would be safer but would take a considerable time to remove all the endangered civilians, and time is short. The US navy’s Ospreys can only carry a small number of people at any one time. Aid and human rights officials say they stranded population, estimated at being between 20,000 to 30,000 cannot survive on air drops alone for much longer.

The US ran a military and intelligence base on a now disused-airfield in the area for much of the Iraq war and the terrain of the rugged 45 mile ridgeline is well known to special operations units. The airfield could be used as one end of an air bridge to fly refugees to safety, if it was impossible to open a land route.

Kieran Dwyer, a spokesman for the UN office for the coordination of humanitarian affairs, said: “It’s really hot up on the mountain. It’s pushing 45 degrees [C] in the shade, and there is very little shade up there. The people are really exposed, and they ran there quickly without taking much with them, so they are in a really tough situation.”

Dwyer, speaking from Irbil, the main city in Kurdistan, said that the Yazidis were spread over multiple locations across the bare mountain. Those towards the north had a means to walk towards Syria to escape, but he said the walk of at least seven hours was gruelling and dangerous as militants could reach part of the route. Those on the south side were more completely hemmed in by Isis militants.

The White House insists that defending its forces against attack from Isis during an evacuation mission would be different from seeking out an engagement with the militants, which it is leaving to others.

The Yazidi refugees who have managed to escape the mountain on their own continue to stream across the northern border into Syria, and then into Iraqi Kurdistan. The vast majority of those who have made it to safety have endured a seven-hour trek in grueling heat. Nearly all those to have escaped have done so from the northern flank of Mount Sinjar, which was cleared of Islamic State jihadists over the weekend.

The south, though remains besieged, with diplomats in Irbil and senior Kurdish officials acknowledging that efforts to clear the road of jihadists has failed. Masrour Barzani, the chancellor of the Kurdish region security council told the Guardian that 170 peshmerga forces had been sent towards the area in an attempt to reach entrapped minorities, however many more would be needed to safely bring them down from the mountain.

Officials at the Feshkhabour crossing from Syria say at least 1,000 Yazidis crossed into Kurdistan on Wednesday, adding to the estimated 40,000 who have made the journey since Sunday. Nearly all are now sleeping rough between the border and the city of Duhok, around 100km away.

Exhausted families sheltered in hedge lines just over the bridge that led them to safety, some sobbing and others clinging to infants and elderly men and women who collapsed onto whatever grass they could find. Local officials handed out water to the new arrivals. Some were carrying bread handed out by NGOs.

“At least six people have died in transit due to dehydration and exhaustion in the last three days, and many more have reportedly passed away in the besieged areas of Sinjar,” said Dr Gustavo Fernandez, programme manager from Médecins Sans Frontières. “With the situation increasingly critical throughout the country, access to people who are trapped in conflict areas is impossible.”

• This article was corrected on 13 August 2014. The original said more than 100 US forces landed on Mount Sinjar; in light of later information this was amended to state that the team was smaller.